Many processes and devices have been used in image processing for digital printing. The acceptance of electrostatic print engines by the graphic arts and commercial print market has heretofore been hampered by various problems, including the lack of acceptable uniformity appearance, especially in the mid-tone range. In the image forming apparatus of the electro photo-graphic system, an apparatus capable of outputting a color image by arrangement of a plurality of image forming units corresponding to color components, color-decomposed in accordance with subtractive primaries or repetition of an image forming process, has previously been put to practical use.
Image noise is among one of the top image quality issues for electrostatic printing when compared to offset printing. Noises at high and low spatial frequencies are typically referred to as graininess and mottle within the visual frequency range. The image noises at various spatial frequencies, namely graininess & mottle, are of particular interests to this application. In particular, the relative amplitudes of the image noises at different frequencies are adjusted to optimize the visual uniformity of a print.
Ideally, it is desired to eliminate the image noise at all frequencies. But current electrostatic printing technology still leaves a significant amount of noises at various frequencies, which leads to quite objectionable graininess and mottle (blotchy) appearance.
Visual masking is a well known phenomenon that some visual effects are less noticeable in the presence of some other more visually dominant features. Some image processing methods are taking advantages of this phenomenon by using noises to mask out moiré, banding and contouring. Another particular example is the visual masking of mottle by graininess: image mottle (low frequency variation) perception improves as the image graininess increases. Therefore, there is an opportunity to optimize the overall uniformity (combining both mottle & graininess) by adjusting the levels of noises at different frequencies. Although it is difficult to lower the system noise, it is possible to increase the noise levels at pre-selected frequencies to achieve the desired visual optimization.
Image mottle is particularly troublesome for some systems. Following the visual masking principle, adding noise to the image, especially high frequency noises, such as graininess noise, have shown to improve the mottle appearance of the image. Due to the random nature of noise, noise addition to the print has only been demonstrated through manipulation of the input contone images before halftoning.
Although the addition of noise to images has show improvement in mottle appearance, such methods are still inadequate. What is needed are better and more flexible methods to improve uniformity appearance of images.